The present invention relates to a wireless communication system, and more particularly to a wireless communication system by the utilization of a WLL (wireless local loop) system.
Recently, attention is paid to the WLL system in constructing communications facilities because it has the following advantages. The WLL system provides quick and economical construction of communication facilities. The WLL system is capable of flexibly complying with various demands and requirements. The WLL-based communication facility is easy in its maintenance and is hard to be damaged in a disastrous event.
The WLL system replaces a conventional wire access equipment, which uses metallic wire cables, with a wireless access equipment. The access equipment connects end users as subscribers to a node point owned and managed by an enterpriser concerned. When the radio access equipment is employed, an access network capable of providing a subscriber telephone service can be constructed economically and for short time. Further, maintenance of the resultant access network is easy, and is capable of flexibly complying with various demands and requirements by the subscribers.
Technique on the WLL is disclosed in JP-A1-8-506471 and JP-A-6-177832. A system diagram showing a conventional WLL is shown in FIG. 5. In the figure, a public switched telephone network (PSTN) 105 is connected to a wireless access network 111111a and 111b) through subscriber""s lines. Those wireless access networks 111 are connected to digital wireless transceivers RP101 (101a to 101c) as shown.
Each digital wireless transceiver RP101 is a transceiver which covers a relatively narrow range of communication. The transceiver is wirelessly linked to wireless fixed access units 107 (107a, 107b) or portable, wireless telephone sets 110 (110a, 110b). The wireless fixed access units 107 are connected to home telephones, as shown. Thus, the conventional technique realizes the wireless connection of home telephones to the public switched telephone network by use of the wireless fixed access unit 107.
The WLL by the conventional technique is able to connect the mobile subscribers as well as the fixed subscribers to the public switched telephone network (PSTN). In this case, the mobile subscribers are placed in various limits on their moving area. This is not essential to the present invention, however, and hence no further description will be given here.
The conventional technique will be described on the WLL system applied to a microcell basis wireless communication system, such as a PHS system (this WLL system=PHS-WLL). The PHS (personal handyphone system) is a simplified mobile phone system standard (RCR-STD 28) which is realized in Japan, and employs a TDMA (time division multiple access) system. The PHS-WLL is an example of WLL of the TDMA system using air interface of the PHS.
In general PHS-WLL systems, a communicable distance (distance between a base station and a subscriber) is 3 to 5 km for the fixed subscriber, and approximately 100 m to 500 m for the mobile subscriber. These figures show that a communicable area of the mobile subscriber is only in the vicinity of the base station. In other words, to enlarge the communicable area of the mobile subscriber, the necessity is to install the base stations at intervals of 200 m to 1000 m (two times as long as the communicable distance or range). In this case, the fixed subscriber accommodates cells each of 200 m to 1000 m.
The following problems arise in a telephone network where the mobile subscribers and the fixed subscribers are accommodated in one base station.
(1) A number of connections existing in the range from a telephone exchange to a base station is increased since the number of base stations installed is increased. Use of the wires for those connections will deny the basic concept of the WLL system, which is directed to reduction of the number of wires used.
(2) An attempt, generally employed, to increase the speech quality of the fixed subscriber is to increase the quality in the wireless path by use of a directional antenna of the radio set of the fixed subscriber. However, radio interference between and among adjacent cells rejects the attempt, and even if so done, the speech quality guaranteeing is impossible.
(3) When a number of calls are directed to a cell, a call originated by the fixed subscriber fails to seize the radio channel, resulting in failure of speech communication.
An object of the present invention is to provide a radio communication system which provides efficient construction of wireless communications facilities by locating subscribers of a wireless local loop and a base station for microcell-basis, portable communication terminals within a communication area.
The present invention provides the following solutions to the problems mentioned above:
(1) A base station accommodating fixed subscribers has a communication area of 3 km to 5 km in radius, and is arranged as in the ordinary PHS-WLL. The speech quality of the fixed subscribers is improved by use of a directional antenna for the fixed subscriber""s radio sets, as a matter of course.
(2) A base station for mobile subscribers has a communication area of 100 m to 500 m in radius, and is arranged as in the ordinary PHS system.
(3) The base station for mobile subscribers is wirelessly linked to the base station for fixed subscribers, and the former serves as a repeater and has a major function of frequency conversion.
(4) The radio frequencies allocated to the fixed subscribers are different from those allocated to the mobile subscribers, eliminating radio interference therebetween.
(5) The traffic intensity of the base station for fixed subscribers is large, and then the maximum simultaneous communication capability (number of speech channels) is increased.
(6) In the base station for fixed subscribers, all the channels are fixedly assigned to the fixed subscribers and the mobile subscribers, to thereby give rise to no deterioration of the switching performance of the fixed subscribers even if a number of calls are directed to a cell.
It should expressly be understood that the numerals indicative of the radium of the communication area are those of an ordinary PHS system, and is presented to by way of example, and those limits the concept of the present invention in any way.